The present invention relates to a semiconductor memory, and more particularly to a semiconductor memory suitable for micro-computer system.
Semiconductor memories have been widely utilized in the micro-computer system.
With the conventional memories, the data that can be read or written at one time or one cycle are fixed to a bit width of an input/output (I/O) port of the memories. For example, the bit width of the conventional memories is typically 1 bit or 4 bit, and therefore the data accessed at one time or one cycle is limited to 1 bit or 4 bit. While, the bit width of data bus in the micro-computer system is larger than the bit width of the memories, and is usually 16 bit, or 32 bit.
Accordingly, when the data exceeds the bit width of the I/O port, a latch circuit is provided as an external unit, and the data read from the memory held by the latch are transferred to peripheral device via an additional wiring.
Therefore, with the micro-computer system provided with the conventional memory, data necessitated by the micro-computer are read from the memory through a plurality of read operations or read cycles. However, the read data obtained through the plurality of read operations involve errors when the data are changed during this lengthy reading period, e.g., when a carry has occurred during the period in which the data of a timer circuit are written into the memory. When the data in excess of the bit width of the I/O port of the memory, is to be read-out, the data must be once latched before they are transferred to the micro-computer, or the input to the memory must be inhibited while the data are being transferred in order to prevent the data from changing. Moreover, system which employs the latch circuit to read the data from the memory only but does not work to write the data into the memory.
Furthermore, use of the latch circuit and the additional wiring results in an increase in the required area on the semiconductor substrate, and therefore limitation is imposed when the system including the memory is to be laid out on the same substrate.